This invention relates generally to acoustic transducers and, more particularly, to an improved composite transducer array having tunable, impedance-controllable and electronically-addressable multilayer sensor-actuators within a housing structure.
Actuating elements, whether crystals, ceramics or polymers, have been employed in a variety of devices including microphones, ultrasonic devices, accelerometers, hydrophones and oscillators. When power density is a primary goal, and the characteristic impedance of the medium is relatively high, e.g., water, ceramic actuators are typically employed. Ceramic actuators are typically either piezoelectric or electrostrictive, each of which have distinct operating characteristics. For example, piezoelectric actuators have a linear response when an electric field is applied, whereas unbiased electrostrictive materials have a quadratic response to an electric field; piezoelectric materials are polarized during fabrication so sensitivity (to pressure, strain, or electric field) is pre-set, whereas electrostrictive materials are polarized when a DC bias is applied and thus sensitivity can be varied during operation; piezoelectric materials operate below the Curie Temperature (T.sub.c) and as the operating temperature approaches T.sub.c, the material degrades in a non-recoverable manner, whereas electrostrictive materials operate within a temperature range close to T.sub.c and above. Therefore, although both piezoelectric and electrostrictive actuators do change shape when an electrical field is applied they are very different and both have distinct advantages and disadvantages. Commonly assigned U.S. Pat. No. 5,023,032, entitled "Electrostrictive Ceramic Material Including a Process for the Preparation Thereof and Applications Therefor", to Bailey et al., describes an electrostrictive actuator composition which has both a relatively wide operating temperature range and a high electrostrictive constant and therefore produces increased strain (displacement) over prior art compositions. This patent, including all references contained therein, is hereby incorporated by reference in its entirety.
Generally speaking, the electrostrictive effect in an actuator comes from the direct attraction and repulsion of anions and cations in the crystal lattice, resulting in a physical distortion of the lattice responsive to the application of an external electric field. This lattice distortion causes a displacement, or strain, in the material. Stated quantitatively, the strain in electrostrictive materials is proportional to the electrostrictive coefficient multiplied by the square of the electric polarization. Therefore, the strain is directly related to both the square of the dielectric constant and the applied electric field. As stated above, the strain in piezoelectric materials is equal to the piezoelectric coefficient multiplied by the electric field.
Ceramic actuators typically have electrodes on opposed top and bottom surfaces for applying the electric field. Once the actuating material is chosen, e.g., piezoelectric, electrostrictive, etc., the principle way to increase the displacement is to increase the amount of actuator material between the electrodes. However, as the distance between the electrodes is increased the electric field per voltage unit decreases. Thus most ceramic actuators must work at extremely high voltages to both increase the amount of material between the electrodes while keeping the electric field high enough to create sufficient strain.
This dichotomy between obtaining high displacement and maintaining low voltage complicates the implementation of distinct actuating elements into a system. There have been systems having a plurality of actuators that require a high individual driving voltage, but they require the power supply and controlling electronics to be separated from the transducer assembly. For example, one of the most common uses of piezoelectric elements is in underwater sonar equipment in which sonar transducers are stimulated by electric signals to emit sonar (acoustic) signals which radiate out from the transducers. The sonar signals are reflected from underwater objects and the reflected signals are received by the transducers, which produce electrical signals carrying information about the underwater objects. The transducer assemblies consist of relatively thick ceramic plates (up to about 1 centimeter) and require drive voltages on the order of several thousand volts. Another example of implementing piezoelectric actuators in a system is in the medical imaging field. Tissues of a human body can be imaged by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that are caused to travel into the body. Echoes from the tissues are received by the acoustic transducer elements and are converted into electrical signals. These signals are amplified and processed to form a cross-sectional image of the tissues. The elements in these medical ultrasound arrays are very small because of the high operating frequencies leading to an electrical impedance mismatch problem with the three to six foot cable connecting the electronics.